MOLECULAR-DYNAMICS ANALYSIS OF CHARGE FLUCTUATIONS ASSOCIATED WITH FAR-INFRARED ABSORPTION IN WATER

Far-infrared absorption spectroscopy gives information on the charge f luctuations associated with hydrogen-bond dynamics in water. Here, we examine these fluctuations by calculating spectra from molecular dynam ics simulations and comparing with experiment. Permanent and induced d ipoles are included in the calculation of the system dipole moment. Th e induced dipoles are calculated on each atom using an iterative metho d that leads to self-consistency of the induced dipole contribution to the local held. The experimental far-infrared spectrum has two promin ent bands, at similar to 600 cm(-1) and similar to 200 cm(-1), due to hydrogen-bond libration and stretching, respectively. The librational band is found to arise from fluctuations of the positions of the perma nent charges in the simulation, whereas the stretching band originates from induced dipole fluctuations. The use of the self-consistent meth od enhances the intensity of the stretching peak relative to a non-ite rative method, producing improved agreement with experiment. The frequ ency of the librational band is influenced significantly by long-range electrostatic interactions in the potential function. In contrast, th e induced dipole fluctuations are relatively insensitive to the long-r ange effects; the effective isotropy of the molecular polarizability t ensor is found to lead to a decoupling of the induced dipole fluctuati ons from the dynamical intermolecular orientational correlations of th e permanent dipoles.